Capacitive touch sensor devices have been incorporated into a variety of consumer electronics, including cellular telephones, computers, portable entertainment devices, appliances, and touch screens, to name a few. At a minimum, a capacitive touch sensor device includes one or more sensors (or “electrodes”), a charging circuit, and a touch detection circuit. Each sensor may be associated with a possible user input. For example, to enable a user to make a telephone call, a cellular telephone may include an array of at least twelve sensors, with ten of the sensors being associated with each of the numbers from 0 to 9, an eleventh sensor being associated with a “SEND” key, and a twelfth sensor being associated with an “END” key.
A typical sensor includes a dielectric touch plate (e.g., a glass plate) and an electrode, which function as a dielectric and an electrode of a capacitor, respectively. When a user touches the touch plate at a sensor location, a potential variation in the electrode is produced due to a capacitive circuit formed between the Earth, the user, and the sensor. The capacitive touch sensor device periodically and frequently measures the potential of the electrode in order to determine whether or not a touch has occurred. Prior to measuring the potential, the charging circuit charges the electrode by providing the electrode with a pre-determined current for a pre-determined time. At the culmination of the charging process, the touch determination circuit measures the voltage between the electrode and ground (or some other fixed potential). When the measured voltage falls sufficiently below a baseline value associated with a no-touch condition (e.g., below a touch detection threshold), the touch determination circuit may indicate that a touch has been sensed. The charging and measurement procedures continue to be repeated, and when the measured voltage later rises toward the baseline value by a sufficient amount (e.g., above a release detection threshold), the touch determination circuit may then indicate that a release has been sensed.
The theoretical range of charges that can be applied to the electrode is constrained to correspond to measurable voltages between zero volts (or ground) and the supply voltage to the device. However, accurate charging and measurement typically can be obtained only for voltages that fall within a central range of the theoretical range. For example, for a device that has a supply voltage of 1.7 volts, reliably accurate voltage measurement may not be possible for voltages within the lower 0.7 volts of the range, and reliably accurate charging may not be possible to achieve voltages within the upper 0.7 volts of the range. Accordingly, accurate charging and measurement may be achievable only for measurable voltages within the central 0.3 volts of the theoretical range.
System designers desiring to incorporate such a capacitive touch sensor device within a product should, therefore, set the charging parameters (e.g., a supplied current and charging interval) so that the charging process results in a voltage that falls within the central range. However, capacitance changes in the system due to external factors (e.g., temperature or humidity changes, transmission frequency changes, and so on) may cause a touch detection threshold or a touch release threshold to move outside of the central, accurately measurable range of voltages. When a threshold moves into the lower portion of the voltage range (e.g., the lower 0.7 volts), accurate measurement may not be possible. When a threshold moves into the upper portion of the voltage range (e.g., the upper 0.7 volts), accurate charging may not be possible. In either case, false touch detections or releases may occur and/or the system may fail to detect actual touches or releases.
Because of these characteristics of conventional capacitive touch sensor devices, system designers typically set charging parameters to produce voltages within the center of the central range (e.g., at 0.85 volts for a 1.7 volt system). Although this practice may increase reliability of the device, device sensitivity is not optimized. In addition, semiconductor device variations may significantly affect the operation of current supplies and timers associated with the charging process. Thus, yields for capacitive touch sensor devices should be low enough to ensure that supplied devices are capable of performing charging process that result in measurable voltages substantially at the center of the range.